1. Field of the Invention
The present invention concerns a speed control device of a motor.
2. Related Background Art
In a speed controller according to the prior art, as shown in FIG. 5 of the accompanying drawings, a motor unit for driving a light amount control member has a differential amplifier 1 for comparing a speed setting voltage with a speed control signal, a driving coil 3 for rotating a rotor magnet 4 by a signal from a power amplifier 2 for receiving as an input a speed error signal output from the differential amplifier 1, a braking coil 6 for detecting and controlling the rotational speed of the rotor magnet 4 for driving the light amount control member, and a magneto-responsive element 5 for detecting the aperture value. An output signal from the braking coil 6 is input as a speed control signal to the differential amplifier 1 through a signal amplifier 7 and a signal from the magneto-responsive element 5 is output as an aperture value detection signal through a signal amplifier 8.
The braking coil 6 for detecting the rotational speed of the rotor magnet 4 has been used for the detection of the driving speed of the light amount control member. Also, the braking coil 6 utilizes the counter electromotive voltage from the rotation of the rotor magnet 4 and therefore has been very low in sensitivity. Further, to increase the sensitivity, the number of turns of the braking coil 6 must be made very large with the (result that the size of the motor unit becomes large, and in order that the size of the motor unit may not become large, it has been necessary to wind the coils around a brakable thin conductor.
Also, in a system for detecting the rotational speed of the rotor magnet by the braking coil, there has been, in addition to the counter electromotive voltage by the rotor magnet 4 being utilized for the braking coil 6, a problem that the mutual induction by the driving coil 3 adjacent thereto affects the rotational speed detection signal thereof to readily cause oscillation.